FR2574180A1 - METHOD AND DEVICE FOR DETERMINING THE ANGLE OF CONTACT OF A DROP OF LIQUID PLACED ON A HORIZONTAL SOLID OR LIQUID SUBSTRATE - Google Patents

Abstract

A SUPPORT 56 IS CLEAN TO HOLD HORIZONTAL A SUBSTRATE 10 ON WHICH A DROP OF LIQUID IS LAID. A SOURCE OF LIGHT 74 ASSOCIATED WITH A COLLIMATOR 78 FORMS A PRIMARY BEAM OF PARALLEL LIGHT, WHICH IS DIRECTED 86 PERPENDICULAR TO THE SUBSTRATE. A RECEIVING SURFACE 126, 128 IS PLACED IN A KNOWN POSITION WITH RESPECT TO THE SUBSTRATE, TO INTERCEPT IN A KNOWN GEOMETRY, THE SECONDARY BEAM 124 PRODUCED BY INTERACTION OF THE PRIMARY BEAM WITH THE DROP AND THE SUBSTRATE. FROM THE ILLUMINATED AREA OF THIS SURFACE, THE VALUE OF THE ANGLE OF CONTACT OF THE DROP WITH THE SUBSTRATE IS DRAWN.

Description

25741 80

  ANVAR Aff. 15 i Method and device for determining the contact angle of a drop of liquid placed on a horizontal substrate

solid or liquid.

  The present invention relates to a method for determining the contact angle formed by a drop of liquid with a horizontal substrate, solid or liquid, on which it is placed, as well as a device for carrying out this method. Contact angle measurements are commonly used to characterize the surface properties of solid or liquid materials. We know that the value of

  the contact angle between a drop of liquid and the

  solid or liquid trat on which this drop is placed depends on the forces prevailing at the liquid-solid or liquid-liquid interface.

  These measures are applicable not only with subs-

  liquid trats to measure the interfacial tension between two liquids, but especially with solid substrates

  to measure their interfacial properties and therefore

  quent their wetting properties.

  It is known in fact that, in the case of perfect or complete wetting, the drop of liquid spreads over the substrate and the contact angle is then zero, while, in the case of partial wetting, this contact angle is then understood

between 0 and 180.

  If we take the example of a drop of liquid water,

  rated by its vapor, and placed on a solid substrate, we know that the contact angle 0 on the border line of the three

  phases, or triple line, is a single value for over-

  smooth, homogeneous and isotropic surfaces of the substrate. Its value is linked by Young's equation: SV 1SL YLV cosO

  to free energy ySv at the solid-vapor interface, to the

  free voltage at SL at the solid-liquid interface and at voltage YLV at the liquid-vapor interface. Remember that Zisman

  emphasized the concept of critical surface tension

  as for wetting, YC, which is a characteristic of each solid surface. Thus, glass and metals are examples of high energy surfaces, on which most liquids spread spontaneously, the angle O

  then tending towards zero. On the contrary, plastic materials

  that such as tetrafluoroethylene are typical low energy surfaces, so that the liquids deposited on these surfaces remain in the form of droplets having

  finite contact angles as long as yC is less than yLV.

  These contact angle measurements are used not only

  in fundamental or applied research, but also in the

  industry to perform routine tests on the states of

surface of certain materials.

  These measures are of interest to a large number of companies

very different activities.

  This applies to companies that carry out

  surface condition tests on different materials,

  sockets working on plastic and polymer films, surfactants and detergents, textiles, inks,

  adhesives, composite materials and special materials

  materials, for example biomedical materials, contact lenses, tooth products, etc. Also affected are companies working on

  thin film deposits to modify the properties of

  face of a given material, for example the deposit of tetra-

  thin layer fluoroethylene on metal drums

  used in offset printing. Other industries are also

  concerned, such as the oil industry.

  There are currently a number of processes

  tests to measure contact angles.

  Most of these methods are based on the observation of a drop placed on a substrate, which has the advantage of

  only ask for small amounts of liquid and over-

  substrate faces less than a few square millimeters.

  In the most widely used methods, the image of the drop is projected "in silhouette" and the angle of the drop is measured point by point using a telescope and a

  protractor, which requires a visual estimate of the tan-

  kind. The reproduction obtained with such processes is low--

  ble and certainly less than 1 2 when comparing the data obtained either on successive drops or

  on several views in projection of the same drop.

  This lack of reproducibility is further exacerbated by heterogeneity.

  roughness and local roughness of solid substrates used

  his. Since this problem is inherent to the substrate itself, it will always be present regardless of the care

  brought in by the experimenters. The only way to dimi-

  to reduce the dispersion of the results is then to average over a large number of individual measurements, which

  is obviously a particularly fast task.

godly and delicate.

  In another known method, interfacing fringes are created.

  in a corner of liquid formed by the edge of the drop

  and the contact angle of the interfringe is deduced therefrom. This pro-

  however, can only be applied for the determination

  tion of very small contact angles. Another measurement method is also known which consists in performing a self-collimation of light very locally on the edge of the drop, by means of a beam of light.

  whose angle of incidence is varied with respect to the

  trat. The intensity of the beam reflected by the drop is observed.

  te in order to detect the moment when the extinction of the

  says beam. The position of the angle of the incident beam makes it possible to immediately deduce the contact angle of the drop with the substrate. The disadvantage of this method is that it uses, like the previous ones, a measurement on a particular section of the drop, which again leads

  poor reproducibility of the measurements.

  The process which is the subject of the present invention makes it possible to avoid the abovementioned drawbacks because the contact angle between the drop placed or "sessile" and the substrate is determined simultaneously over the entire periphery of the drop. The determination of the contact angles is then carried out quickly and precisely, the precision being typically 0.1 instead of 1 to 2Q with the methods of the prior art. The method also allows an objective measurement, because it does not

  not require a visual estimate of a tangent to the pro-

  drop wire at the point of contact, so that the deter-

  minations are not likely to vary from observation

to the other.

  The method of the invention does not require an optical microscope as with most of the prior methods; he

  allows immediate visual detection of ill-formed drops

  which would have been impossible with an earlier process

  and would have given rise to an incorrect measurement.

  The method of the invention also constitutes an immediate test.

  diat of the homogeneity of the substrate from the form of

  gout (asymmetry, irregularity of the outline).

  In addition, the method allows automation and measurement

  continuously, it being observed that this measure is particular-

  simple to implement, as will appear in the

  read the description below.

  The method for determining the contact angle of a drop of liquid placed on a horizontal, solid or liquid substrate, according to the invention, essentially comprises the operations consisting in:

  a) direct a primary beam perpendicular to the substrate

  mayor of parallel light suitable for illuminating an area which includes the interface between the periphery of the drop and the substrate, this primary beam interacting with the drop and the substrate to produce at least one secondary beam;

  b) measure the extent of the lighting transition that pre-

  feel the secondary beam, corresponding to the peri-

  gout of the drop, on at least one receiving surface

  capturing this beam in a position with respect to the

  known trat and geometry, which makes it possible to determine the contact angle from said measurement as well as from the position and geometry

  obtained from said receiving surface.

  In a first embodiment of the invention, the secondary beam is produced by interaction of the primary beam with the drop and the substrate, the drop acting

then like a convex mirror.

  In a first embodiment of the invention, the

  this primary element may be directed towards the face of the substrate on which the drop is placed. or else it may be directed towards the face of the substrate opposite to that on which is placed the

  drop, the substrate then being transparent.

  In the case where the primary beam is directed towards the face of the substrate on which the drop is placed, the beam

  secondary can be produced in three different variants

  annuities that depend on the ranges of contact angles and

the nature of the substrate.

  In a first variant, the secondary beam is pro-

  duit by a simple reflection of the primary beam on the convex surface of the drop, the nature of the substrate then being indifferent, which makes it possible to determine angles

contact between 0 and 45.

  In a second variant, the secondary beam is pro-

  duit by a reflection of the primary beam on the convex surface of the drop, followed by a transmission through the substrate, the latter being transparent, which makes it possible to

  determine contact angles between 45 and 90.

  In a third variant, the secondary beam is pro-

  duit by a reflection of the primary beam on the surface

  convex of the drop, followed by a reflection on the subs-

  trat, the latter acting as a plane mirror, which

  puts to determine contact angles between 45

and 90.

  In the first embodiment and in the case where the substrate is transparent and the primary beam is directed towards the face of the substrate opposite to that on which

  the drop is placed, we can determine the angles of

tact between 90 and 1800.

  According to a second embodiment of the invention, the secondary beam is produced by interaction of the primary beam with the drop and the substrate, the drop acting as a convex plane lens and the substrate being

  transparent, which makes it possible to determine angles of con

tact between 0 and 90.

  The receiving surface used to implement the pro-

  ceded of the invention may in particular be a flat surface arranged parallel to the substrate, a generally cylindrical surface whose axis is directed perpendicular to the substrate and passes through the center of the drop, or else a hemispherical cap-shaped surface centered on the

  drop, and whose equatorial line is placed on the horizon-

  tale. The use of a receiving surface in the form of a hemispherical cap is particularly advantageous since it allows a direct reading of the contact angle, if this hemispherical cap is provided with a graduation

appropriate angle.

  The receiving surface can also be part of a provision

  automatic detection, for example a video camera

  coupled with a computer, or even an optical detector

  electronic, photoresistor type, if possible with response

strongly non-linear.

  The invention also relates to a device for setting

  implementing the method, this device comprising essential-

  a suitable support to maintain a horizontal substrate-

  ment, a light source, means to form from

  shooting from the source a primary beam of parallel light, means for directing this beam perpendicular to the substrate and at least one receiving surface in a position

  with respect to the substrate and a geometry known for inter-

  cepter the secondary beam produced by interaction of the

  primary beam with drop and substrate.

  The means used in the device to direct the

  the primary beam perpendicular to the substrate advantageously comprise orientation means for directing the primary beam either in the direction of the face of the substrate on which the drop is placed, or on the contrary in

opposite direction.

  Other characteristics and advantages of the invention result from

  will be from the examination of the detailed description which follows,

  as well as the accompanying drawings, in which:

  - Figure 1 is a schematic representation of a dis-

  positive implementing the method of the invention to determine the contact angle of a drop of liquid placed on a substrate;

  - Figure 2 is a detail of Figure 1 showing more par-

  particularly the arrangement of the primary beam and the secondary beams, reflected and refracted by the drop of liquid; - Figures 3 to 6 are figures similar to Figure 2 showing the arrangement of the primary beam and the secondary beam for different modes and variants of implementation of the method;

  - Figure 7 is a top view of a device

  measurement according to the invention; - Figure 8 is a sectional view taken along the line VIII-VIII of Figure 7; - Figure 9 shows a refracted "image" obtained with

  a device according to the invention; on a receiving surface;

  flat trice of the photographic plate type;

  - Figure 10 is a "refracted image" obtained in the same

  my conditions as the "image" of figure 9, but the needle having been used to deposit the drop having remained in place during the measurement;

  - Figure 11 is a graph showing the variation of the co-

  sine of the contact angle O as a function of the voltage

  liquid-vapor terfacial for a series of n-alkanes with

  room temperature, from a series of measurements taken

  killed according to the invention on a polytetrafluoro-substrate

ethylene; and

  FIG. 12 is a graph obtained under the same conditions.

  as the graph in Figure 11, but using a borosilicate glass plate covered as a substrate

a monolayer of hexadecanol.

  FIG. 1 shows a horizontal substrate so-

  the liquid constituted here by a glass plate 10 which can for example be a microscope plate. On the upper face of the plate 10 is placed a drop of liquid 12 forming a contact angle 0 with the substrate (see FIG. 2). As shown in FIG. 1, a light source 14, for example suitable for emitting a laser beam, emits a parallel and horizontal beam of light whose diameter is dilated by means

  two lenses 18 and 20 arranged in afocal configuration.

  The diameter of the beam 16 is thus increased to form a

  beam 18 which is directed on a mirror 20, arranged in a form

  mant an angle at 45 with respect to the horizontal, so as to emit a primary beam 22 of parallel light in the direction of the substrate 10. This beam of light must have such a diameter and an orientation that it can illuminate an area which includes the interface between the periphery of the drop and the substrate. In practice, it is arranged so that the beam 22 is directed vertically and centered on the drop, while having a diameter greater than the diameter Dd of

drop.

  By way of example, the source 14 can be a heliumneon laser source having a wavelength of 632.8 nanometers and suitable for emitting a laser beam having a diameter of 1.5 mm. This bundle can be expanded so as to have

  for example a diameter of 1 centimeter, i.e. a diameter

  sufficient meter to be able to fully encompass a drop

  which one wishes to measure the contact angle with a substrate.

  As will be described later with reference to FIGS. 7 and 8, it is also advantageous to provide means for

  that the primary beam 22 can be directed towards the sub-

  trat, but towards the face of the latter which is opposite to that supporting the drop. In this way, we realize a lighting of the drop from below, as it will

shown below.

  The primary beam 22 interacts with the drop 12 and the

  substrate 10 for producing at least one secondary beam.

  In fact, the surface of the drop acts like a mirror

  vex which reflects a secondary beam 24 constituted by a cone of light whose opening angle is equal to 46,

  the contact angle at the periphery of the drop being greater

  set constant (see Figure 2). The reflected image is inter-

  ceptée on a receiving surface of known geometry and placed

  created in a known position relative to the substrate. This makes it possible to measure the extent of the illumination transition presented by the secondary beam 24, in correspondence

from the periphery of the drop 12.

  As shown in FIG. 1, the image is intercepted on a receiving surface 26 constituted by a screen arranged horizontally and at a distance h from the substrate. We obtain

  thus on this screen a bright spot, generally circu-

2574-180

  laire, which has a diameter D. We can easily show that the diameter D is related to 0 by the relation: D - Dd tg 2 O = 2h We can thus, knowing the value h, and after measuring the

  values Dd and D, obtain the value 8 sought.

  As a variant, it is possible to intercept the secondary beam.

  24 on a generally cylindrical surface 28 whose axis is directed perpendicular to the substrate and passes

through the center of the drop.

  In another variant, which will be described later, the sur-

  s15 receiving face may consist of a spherical cap

  that whose center coincides with the center of the drop.

  The secondary beam 24 is produced by a simple reflection

  xion of the primary beam 22 on the convex surface of the drop which acts as a mirror, the nature of the substrate then being indifferent. This makes it possible to determine angles

contact between 0 and 45.

  Apart from the fact that the drop 12 constitutes a mirror, it

  can also act as a plano-convex lens if the subs-

  trat 10 is transparent with respect to bundle 22. In this latter

  To deny this, a refracted beam 30 can be produced which is intercepted on a receiving surface 32, constituted by

  example by a photographic plate, arranged on the horizon-

  tale and at a distance h 'from the face of the substrate on which the drop is placed. Note that the surface 32 must be placed below the substrate 10 and that there is also generally observed on this surface 32

  circular with an outside diameter D '(cf. figure 1).

  If the refractive index n of the liquid forming the drop is known, D 'is related to the value h' and to the contact anal e by the following relationships: tg 8 = sin 8 "(n 2_sin2,") 1 / 2_1 D '+ Dd tg 0 "= 2h' in which 0" is the refracted angle at the top of the drop (see Figure 2). The secondary beam 30 can also be intercepted on the cylindrical surface 28 or on a surface

  spherical cap type receiver.

  In the aforementioned variant embodiment, where a

  secondary beam 30 refracted by the drop, one can detect

  mine contact angles between 0 and 90.

  In the alternative embodiment of FIG. 3, the primary beam 22 is also directed towards the face of the substrate on which the drop is placed. The secondary beam 34 is produced by a reflection of the primary beam 22 on the convex surface of the drop, which reflection is followed

  by transmission through the substrate 10 which is trans-

  parent. This makes it possible to determine the contact angles

  taken between 45 and 90. In this case, the receiving surface 32 must obviously be arranged in the region included under

the substrate 10.

  We can show that the value 8 can be deduced from the re-

  lation tg 2 8 = - (d2h), hence therefore 2 0 = - arc tg (D-d). In this relation, D denotes the diameter of the image on the receiving surface, d the diameter of the drop and h the distance between the substrate and the receiving surface. In the alternative embodiment of FIG. 4, the primary beam 22 is also directed towards the face of the substrate 10 on which the drop is placed. The secondary beam 36 is produced here by a reflection of the primary beam on

  the convex surface of the drop, followed by another reflection

  xion on the substrate 10, the latter acting as a mirror

  plane, which makes it possible to determine contact angles

  taken between 45 and 90. In this variant embodiment, the

  receiving surface 26 must be arranged in the upper region

  less space compared to substrate 10.

  D-d The value 8 is obtained by the relation 20 = v-arc tg (-2),

  D, d and h being defined as above.

  In the alternative embodiment of FIG. 5, the substrate is transparent and the primary beam is directed towards the face of the substrate which is opposite to that on which the drop is placed. In the case of FIG. 5 where the contact angle 8 is between 90 and 135, the secondary beam 38 is reflected in the upper region of the space by

  compared to substrate 10. It is therefore advisable to have an additional

  receiving face 26 in this region of space. The value D-d O is deduced from the relation 2 0 = r + arc tg (-d), D, d and h

  being defined as above.

  In the embodiment of FIG. 6, which closely resembles that of FIG. 5 since the substrate is transparent and the primary beam is directed towards the face of the substrate opposite to that on which the drop is placed, we measure contact angles between 135 and 180. However, in this particular case, the beam

  secondary 40 is reflected in the lower region of the es

  pace with respect to the substrate 10. It is therefore in this lower region that a receiving surface 32 should be arranged. The value O is obtained in this case by the Dd relation 2 = 2- arc tg (-2h), D , d and h being defined as above. Thus, by using the assembly of figure 5 or figure 6, one can determine angles

  contact between 90 and 180.

  The relations giving the values 0, indicated above,

  are established in relation to flat receiving surfaces

  nes, of the screen type. However, in the different modes of

  embodiment described with reference to FIGS. 3 to 6, the sur-

  receiving faces can be made in the same way as

  described above with reference to Figures 1 and 2.

  We now refer to Figures 7 and 8 to describe a

  device according to the preferred embodiment of the

vention.

  This device essentially comprises a frame 42 intended to be placed horizontally by means of adjustable feet fixed to the frame (not shown). This frame 42 is delimited by

  two parallel sides 44 and 46 which extend respectively-

  ment by two non-parallel sides 48 and 50 to form a narrowed portion (Figure 7). This frame 42 then constitutes a

* very thick table in which an opening is provided

  ture 52 of generally circular shape, in the region between the sides 44 and 46, and an open longitudinal slot 54 in the region between the two sides 48 and 50 (cf. FIG. 7). Inside the opening 52 is disposed a support or sample holder 56 adjustable in height and intended to receive a substrate 10 on the upper face

  of which a drop of liquid 12 must be placed.

  ser this drop of liquid, we use a micro-syringe

  58 which is fixed under the frame 42 and which is connected, by the

  end of a flexible pipe 60, at one end of a hollow needle 62 whose other end 64 is bent

  to deposit the drop 12 on the substrate 10. The needle-

  the 62 is pivotally mounted around a horizontal axis 66 and is adjustable in position by means of a needle 68 acting on the vertical displacement of a stop 70 capable of cooperating with

  the needle 62 in the vicinity of its end connected to the

  pick 60. The adjustment of the horizontality of the frame 42 can be

  obtained by means of a spirit level 72 placed in a recess

  dement of the upper face of the frame 42.

  Inside the slot 54 is arranged a source 114, such as a helium-neon laser source, capable of emitting a beam 76 directed horizontally. The beam diameter

  76 is increased with a beam enlarger 78

  taking two lenses 80 and 82 in an afocal configuration. In this way, a beam 84 of light is produced

  parallel. This beam 84 can be deflected in two directions

  different conditions thanks to an optical device designated in

  as a whole by reference 86. This optical device comprises

  takes a vertical tube 88 of square section which is provided in its central region with an opening 90 to allow the introduction of the primary beam 84. The tube 88 is connected at its upper part to a horizontal tube 92 and at its lower part to a horizontal tube 94, both of square section. The connection between the tube 88 and the tube 92 and between the tube 88 and the tube 94 takes place respectively by

  inclined plates 96 and 98 arranged at 45. The tube 92 defi-

  nit a horizontal opening 100 disposed above and in alignment with the region of the substrate intended to receive

  drop. Similarly, the tube 94 defines a horizontal opening

  tale 102 also arranged in alignment and facing

  vis the aforementioned region. In the vicinity of the openings 100 and 102, the tubes 92 and 94 are limited by inclined plates 104 and 106, arranged at 45. Inside the vertical tube 88 is placed, opposite the opening 90, a prism (or a mirror) 108 orientable around a horizontal axis 110 by means of levers 112, so as to emit a beam 114

  which can be directed either upwards or downwards.

  In the case of Figure 8, there is shown the beam 114 in its upward direction. The beam 114 is deflected by a prism 116 disposed at the intersection of the tubes 88 and 92 so as to emit a horizontal beam 118 whose direction is deflected by another prism 120. This prism is disposed inside the tube 92 and opposite the opening 100, so as to emit a primary beam of parallel light 122 directed vertically and in the direction of the drop 12 and the substrate 10. The adjustment of the vertical

  the beam 122 is obtained by screws 123 or the like which pass through the inclined plate 104 and act on the

  support of the prism 120 to modify its orientation.

  Of course, two other prisms, similar to prisms 116 and 120, are disposed respectively at the intersection of the tube 88 and the tube 94 and inside the tube 94 in

look of the opening 102.

  At the exit of the prism 120.1 primary beam 122 possibly passes through a diaphragm 125 before being

  directed at the drop and the substrate.

  The substrate 10 may be a plate, for example a glass plate, having a flat and polished surface for receiving the drop of liquid. It can also be a substrate

  solid, such as a film, or a liquid substrate, such as a thin film

  which is immiscible with the liquid constituting the drop.

  In the following detailed description, it will be assumed that

  the substrate is a glass plate.

  In the embodiment of FIG. 8, the main beam

  mayor 122 is reflected by the convex surface of the drop

  12, which acts as a mirror, to return a secondary beam

  Daire 124 intended to be intercepted on a receiving surface

trice.

  A first receiving surface 126 in the form of a crown cap

  center centered on the drop 12 is arranged above

  above the substrate 42 so that its equatorial line is placed horizontally and comes to bear on the frame 42. The position of the substrate is adjusted so that its upper face is in the equatorial plane of the

hemispherical cap.

  A second receiving surface 128, also in the form of a hemispherical cap centered on the drop, is placed

  in such a way that its equatorial line is placed at the time

  horizontally and resting under the underside of the frame 42.

  The receiving surface 126 in the form of a spherical cap comprises

  has a diameter greater than that of the receiving surface 128, and all these two surfaces are made of a diffusing material and are advantageously provided with graduations allowing the angular opening of the or to be directly

secondary beams.

  The surface 126 is tightly connected to the frame 42 by means of an O-ring 130 and it can be

  removed from the frame using handles 132.

  The receiving surface 128 is also connected in a manner

  tight to the frame by means of an O-ring 134.

  Surface 128 is normally intended to remain in place

  on the frame but can be removed if necessary.

  These two receiving surfaces 126 and 128, thus define

  around the drop 12 an airtight enclosure, which allows

  keeps the drop of liquid in balance with its

  steam. For this purpose, a tank 138 intended to receive a soil-

  Vant to saturate the atmosphere inside the enclosure is provided in the thickness of the frame 42 and in the region

  annular between the opening 52 and the perimeter

  spherical of the receiving surface 126. The tank 138 can be drained thanks to a drain orifice closed by a plug

removable 140.

  If necessary, the device can include means for

  heating to bring the liquid from the drop to a temperature

  rature higher than room temperature, such heating means are already known and used in other

  contact angle measurement systems.

  In an alternative embodiment not shown, the provision

  optic sitive 86 could consist of a simple bent arm formed by the upper part of the tube 88 and by the

  tube 92, which would be pivotally mounted = opening of a horizontal axis-

  tal in the extension of the direction of the beam 76 and the beam 84. This device could then be brought into two different positions, namely a position above

  of the frame and a position below the frame.

  It will also be noted that the device of FIGS. 7 and 8

  could be equipped with different receiving surfaces

  annuities, in particular with receiving surfaces of the flat screen or cylindrical screen type, as described above

  with reference to Figure 1. The receiving surfaces for-

  are also part of a detection device

  automatic, as indicated above.

  The images shown in Figures 9 and 10 are two examples.

  ples of refracted images obtained with a device conforming to that of FIG. 8, but using a flat screen of the photographic plate type, analogous to the screen 32 of FIG. 1, that is to say disposed below the substrate and

parallel to this one.

  FIG. 9 corresponds to the case where the needle 62 has been removed and where the drop 12 has taken its form of undisturbed spherical cap. If we consider this image starting from its center, we can distinguish: - a dark circle 142 corresponding to the projected shadow of the drop, which gives a scale image of the drop; - a brilliant circle 144 corresponding to the impact on the

  photographic plate of the primary beam of laser light.

  It can be seen that, in this particular case, the diameter of the beam is approximately three times that of the drop, this depending on the diameter of the adjustable diaphragm 125; a square 146 delimiting the edges of a neutral density filter, which avoids overexposure of the film by the laser beam transmitted directly;

  - a refracted and dilated "image" 148 of the sessile drop.

  This image makes it possible to directly measure the outside diameter D 'to calculate the angle 0 "and thus the contact angle 6 (cf. FIGS. 1 and 2). The fringes that can be observed are due to diffraction by the edges of the drop and are independent of the actual contact angle at the level of the line of the three phases. These fringes should not be confused with the interference fringes which occur due to the combination of two reflections, respectively

  at the solid-liquid and liquid-vapor interfaces. They can-

  can be easily removed using parallel white light, which slightly improves the accuracy in determining D '. The use of a light source

  white, however, requires a heat filter.

  FIG. 10 corresponds to the case where the needle 62 touches the sessile drop. The contact of the tip of the needle with the

  drop induces deformation of the spherical cap shape

  only gout. In reality, the liquid wets the material forming the needle (stainless steel) and the contact angle with the needle is practically zero. On the other hand, the shape of the drop in the region of contact with the solid substrate is governed by the energies of the three interfaces and not by the shape of the liquid-vapor interface at

  distance from the line of the three phases.

  It therefore appears that the liquid region in the vicinity of the line of contact with the solid remains unaffected by the

  surface deformation caused by the tip of the needle.

  The end result is therefore that we can make the measurement with the needle in place. However, it should be noted that, in this geometry, the angle of incidence between the primary beam, which is vertical, and the normal to the air-liquid interface varies continuously with the position of the beam. It starts from the true value 6 of the contact angle at the line of the three phases. It then decreases towards a nonzero value at the inflection point, whose position

  depends on the magnitude of the disturbance induced by the needle

  the. After this point, it increases again to reach 90

the tip of the needle.

  The properties of the refracted beam follow this variation.

  The angle of refraction takes a value 8 ", connected to 8, at the line of the three phases, goes through a minimum value at the point of inflection and increases again when approaching the tip of the needle. This explains the refracted image observed at

Figure 10.

  Outside the shadow 150 of the drop and the projection

  direct 152 of the primary laser beam on the photo plate

  graphic, the refracted light is concentrated by forming

  a halo 154 bounded by a sharp border 156 on the in-

  térieur and by a diffuse line 158 on the outside.

  It is easy to understand, by considerations of geometric optics, that the sharp border corresponds to the inflection point of the profile of the drop, and that the border

  diffuse corresponds to the 6 "refracted angle along the peri

gout of gout.

  The comparison between Figures 9 and 10 shows that the posi-

  tion of this last line, as indicated by the diameter

  D ', is rigorously equivalent to that observed in the ab-

  contact between the drop and the needle. This confirms the hypothesis that the contact angle O is not modified by the presence of the needle. We draw the consequence that the

  measurements can be performed as well by removing the

  guille having served to put the drop that by leaving this

  needle in place and in contact with the drop.

  In the case of Figures 9 and 10, it will be noted that the images

  drops are practically circular. Characteristically

  ristic, the fluctuations in the drop diameter do not exceed

no more or less 2%.

  However, there may be cases where the circularity is much lower. This is one of the advantages of the invention

  which makes it possible to obtain clearly enlarged "images" for

  easily detect defects in drops which would otherwise-

  escaped the attention of the experimenter. This makes it possible to rule out a priori "images" which can lead to

wrong measurements.

  Reference is now made to FIGS. 11 and 12 which bring together results of contact angle measurements obtained with the method of the invention, on a series of liquid n-alkanes

  (hexane to hexadecane) deposited on hydro glass plates

  phobes and / or on polytetrafluoroethylene plates.

  For these experiments, commercial alkanes of the highest degree of purity available (purity greater than 99%) were used and used without further purification. Commercially available microscope glass plates were used as the substrate. After careful cleaning with a detergent and sulfochromic acid and after rinsing, these plates were covered with a single layer of n-hexadecanol previously recrystallized from ethyl acetate. Alcohols react with hydroxyl groups

  acid surface of silica to form cova-

  slow Si-O-C-X with water release. These bonds are completely stable both thermally and chemically. The hydrophilic surface of the silica is then replaced by a

  dense external surface of methylene groups, which is hydro-

  phobic. This is best demonstrated by the high contact angle presented by water droplets deposited on

this surface.

  Polyurethane lacquers were also used as the substrate.

  commercial tetrafluoroethylene. The surface finish of these plates has been gradually improved using

  sandpaper with an increasingly fine grain. Any-

  this has been insufficient to guarantee good surface conditions. Consequently, this operation was completed by heating the plastic material at 200 C for several hours while pressing it between two surfaces.

planes of good optical quality.

  In these experiments, the drops of liquid were deposited on the plates using hypodermic needles connected to a microsyringe as shown previously in FIG. 8. The evaporation of solvent was minimized by enclosing the whole in a sealed enclosure saturated with solvent vapor. Photographs of the reflected or refracted images were taken after a time interval of from 20 seconds to allow the mechanical balance of the

drop on its substrate.

  The measurements made on the n-alkane series were generally started from the lower counterparts since they are more volatile and easier to eliminate. Between each experiment on a particular alkane, the substrate, the syringe and the needle were carefully cleaned with acetone or toluene. For each liquid, the contact angle measured was the average of at least

  independent measurements on successive drops.

  With such a method, the absolute precision (including the

  reproducibility) was plus or minus 0.25 on the inter-

  experienced angular valley. The data obtained for the different alkanes are shown in Figures 11 and 12 respectively for polytetrafluoroethylene (PTFE)

  and hydrophobic glass substrates.

  With reference to FIG. 11, the variations of the cosine of the contact angle 0 have been reported as a function of the voltage yLV at the liquid-vapor interface of the pure liquid. These measures

  were performed for a series of n-alkanes at-the-temperature

  ambient rature (22 C). On the graph, C6 represents.

  hexane, C16 hexadecane, etc. The solid surface is a polytetrafluoroethylene plate polished as described above. The crosses correspond to the measurements made

  according to the invention, while the solid circles correspond to

  dent in previous data, with error limits of

more or less 2-.

  Using a least squares procedure, we could obtain: cos 8 = - 0.031 LV + 1.555 We deduce that the value LV for which cos 6 = 1 is 18, 0 + 0.2 dynes cm. This value is of particular importance because it defines the minimum surface tension below which a pure liquid spreads spontaneously,

  that is, it forms a zero contact angle.

  The values thus obtained are in good agreement with the literature. With reference to FIG. 12, the variations of

  cos O as a function of the voltage YLV at the liquid interface-

  steam for undecane (C11), dodecane (C12), tetra-

  decane (C14) and hexadecane (C16) at room temperature (20 C). The crosses correspond to the measurements carried out according to the invention. In the case where the solid surface is a borosilicate glass plate covered by a single layer of hexadecanol, we find by a procedure of least

  squares that cos O = - 0.042 yLV + 1.97.

  We deduce that the value of for which cos e = 1, is 23.3 - 0.3 dynes cm 1 On the same graph, the solid circles correspond to previous data with error limits of plus or minus 2 %. The solid surface used is a plate of

  platinum covered by a mono-layer of octadecylamine.

  In both surfaces, the outermost layer is

  a dense concentration of methyl groups.

  The experimental values obtained by implementing the

  process of the invention are in perfect agreement with the values

  their literature. These experiments confirm the validity of the process of the invention and also underline the advantages which it makes it possible to achieve, advantages which have

  have been indicated at the beginning of the description.

  In the device described with reference to Figures 7 and 8, the sample holder-56 is provided for a solid substrate of the glass plate type. We can however, for each type

  particular solid substrate or liquid substrate, pre-

  see a suitable sample holder capable of being mounted, for example by screwing, in the frame 42. The device can thus be adapted immediately to substrates of very different nature.

  Although, in the present description, it has been indicated that

  the secondary beam is intercepted on a receiving surface

  By giving a bright image, it should be understood that this is not a true image of the drop, but rather a representation or angular map of the periphery of the drop. In this regard, it should be noted that the detection of

  possible gout defects, the angle of which is measured

  tact, does not come at all from the dimension of this representation

  sentation or English card. Indeed, even if this representation

  feeling is reduced to a dimension comparable to that of gout, the possible defect will always be much more

  apparent only on gout itself.

  It will be noted that an invisible defect for an attentive observer with the usual methods of determination, becomes obvious

  with the method of the present invention.

257418t

Claims (26)

Claims.   1. Method for determining the contact angle of a drop   liquid placed on a horizontal, solid or liquid substrate   which is characterized by the fact that it includes operations consisting of:   a) direct a primary beam perpendicular to the substrate   mayor of parallel light suitable for illuminating an area which encompasses the interface between the periphery of the drop and the substrate, this primary beam interacting with the drop and the substrate to produce at least one secondary beam;   b) measure the extent of the illumination transition that pre-   feel the secondary beam, corresponding to the peri-   drop sphere, on at least one receiving surface intercepting this beam in a position relative to the known substrate and geometry; which makes it possible to determine the contact angle from said measurement as well as the position and the geometry   known from said receiving surface.   2. Method according to claim 1, characterized in that   the secondary beam is produced by interaction of the   primary skin with the drop and the substrate, the drop acts health like a convex mirror.   3. Method according to claim 2, characterized in that the primary beam is directed towards the face of the substrate on which is the drop.   4. Method according to claim 3, characterized in that the secondary beam is produced by a simple reflection of the primary beam on the convex surface of the drop, which makes it possible to determine contact angles included between 0 and 45.   5. Method according to claim 3, characterized in that the secondary beam is produced by a reflection of the primary beam on the convex surface of the drop, reflection followed by a transmission through the substrate, the latter being transparent, which makes it possible to determine   contact angles between 45 and 90.   6. Method according to claim 3, characterized in that   the secondary beam is produced by a reflection of the beam   primary skin on the convex surface of the drop, followed by a reflection on the substrate, the latter acting as a plane mirror, which makes it possible to determine angles of contact between 45 and 90.   7. Method according to claim 2, characterized in that the substrate is transparent and that the primary beam is directed towards the face of the substrate opposite to that on which the drop is placed, which makes it possible to determine angles.   contact between 90 and 180.   8. Method according to claim. 1, characterized in that   the secondary beam is produced by interaction of the   primary skin with the drop and the substrate, the drop acting as a convex plane lens and the substrate being transparent, which makes it possible to determine contact angles included between 0 and 90.   9. Method according to one of claims 1 to 8, characterized   in that the substrate is a solid substrate, such as a   that or a film having a generally flat surface   intended to receive the drop of liquid.   10. Method according to one of claims 1 to 8, characterized   in that the substrate is a liquid film immiscible with the liquid constituting the drop. 257,180   11. Method according to one of claims 1 to 10, charac-   terized in that the receiving surface is a flat surface   arranged parallel to the substrate.   12. Method according to one of claims 1 to 10, character--   laughed at in that the receiving surface is a general surface   cylindrical, the axis of which is directed perpendicularly   to the substrate and goes through the center of the drop.   13. Method according to one of claims 1 to 10, character-   laughed in that the receiving surface is a hemispherical cap-shaped surface centered on the drop, and the   equatorial line is placed horizontally.   14. Method according to one of claims 1 to 13, character-   laughed in that the receiving surface is a screen.   15. Method according to one of claims 1 to 12, character-   se in that the receiving surface is a photosensitive surface sible or a photographic plate.   16. Method according to one of claims 1 to 10, character-   laughed at in that the receiving surface is part of a dispo-- automatic detection device.   17. Method according to one of claims 1 to 16, character-   laughed at in that the primary beam is a laser beam or a beam of white light.   18. Method according to one of claims 1 to 17, character-   laughed at in that the substrate is arranged inside a   enclosure where the liquid constituting the drop is balanced bre with its vapor.   19. Device allowing the implementation of the method according to   one of claims 1 to 18, characterized in that it   includes a support (56) suitable for keeping a substrate (10) horizontal   a light source (74), means (78) for forming   sea from the source a primary beam of parallel light, means (86) for directing this beam perpendicular to the substrate, and at least one receiving surface (126, 128) in a position relative to the known substrate and geometry to intercept the secondary beam (124) produced by interaction of the primary beam with the drop and the substrate.   20. Device according to claim 19, characterized in that the means (86) for directing the primary beam (122) perpendicular to the substrate (10) comprise means   orientation (108, 116, 120) to direct the primary beam   mayor is towards the face of the substrate on which   the drop is placed, either in the opposite direction.   21. Device according to claim 20, characterized in that   that said orientation means comprise a first socket   me orientable (108) or a mirror capable of emitting, from a horizontal primary beam (84), a vertical beam (114) directed upwards or downwards, at least one second prism (116) capable of deflecting the vertical beam at 90 to bring it to the horizontal, and at least a third prism   (120) suitable for deflecting the horizontal beam (118) for the   lead vertically and form a vertical beam (122) direct to the substrate (10).   22. Device according to claim 21, characterized in that the third prism (120) is adjustable in position by screws (123) or the like.   23. Device according to one of claims 19 to 22, charac-   terized in that it comprises at least one receiving surface   plane (26, 32) arranged parallel to the substrate (10).   24. Device according to one of claims 19 to 22, charac-   terized in that it comprises a receiving surface (28) of   general cylindrical shape whose axis is directed perpendicular   cularily to the substrate (10) and passes through the center of the drop (12).   25. Device according to one of claims 19 to 22,   characterized in that it comprises two receiving surfaces.   (126, 128) each in the form of a spherical cap, arranged respectively above and below the support (42, 56), the two receiving surfaces in the form of a spherical cap defining a sealed enclosure surrounding the substrate (10)   and the drop of liquid (12) placed on it.   26. Device according to one of claims 19 to 25, charac-   terized in that the primary beam (22, 122) is a beam   laser or a beam of white light.